HPF?

[thumbknuckle]

hey there,

Sending pink noise in to the audio injector card and directly back out with no processing applied I am seeing a substantial amount of high pass filtering going on. Things start rolling off at 160Hz and are down just over 24dB after 2 octaves. That's going to be a problem for my intended usage.

fwiw It's ruler flat above that, and latency performance is excellent.

[thumbknuckle]

Is my card broken or is it really supposed to be behaving this way? That's a huge frequency response problem...

[flatmax]

I have done an analysis for you.

I did a loopback with my HPF turned off in alsamixer. I only see a significant roll off above 22 kHz - which is to be expected because of antialiasing with a 48 kHz sample rate. It is VERY flat in the low freq. Here is a screenshot (see below for my alsamixer settings) :

audio injector wm8731 cards - analysis

audioinjector.wm8731.analysis.png (102.12 KiB) Viewed 3546 times

audio injector wm8731 cards - analysis - alsamixer settings

audioinjector.analysis.alsamixer.settings.png (29.49 KiB) Viewed 3544 times

Here is the octave (matlab) script I used for analysis :

Code: Select all

function analysis
% Author : Dr Matt Flax
% Date   : 2017 08 18
% Web    : http://www.audioinjector.net
% Audio injector wm8731 based card analysis : http://forum.audioinjector.net/viewtopic.php?f=5&t=3049&p=5009

[x,fs]=audioread('/tmp/noise.3s.playback.wav');
[y,fsY]=audioread('/tmp/noise.3s.capture.wav');

if fs~=fsY
    error('recordings should have had the same sample rate');
    return
end
clear fsY

trim=10e-3; % trim this much time from the beginning and end to account for latency

y=y(trim*fs:end-trim*fs,:);
x=x(1:size(y,1),:);

Y=20*log10(abs(fft(y))); % dB
X=20*log10(abs(fft(x))); % dB

f=linspace(0, fs, size(Y,1));
Xsm=csaps(f,X,1e-6, f)';
Ysm(:,1)=csaps(f,Y(:,1),1e-6, f);
Ysm(:,2)=csaps(f,Y(:,2),1e-6, f);
fact=max(Xsm)./max(max(Ysm));

clf
subplot(211)
semilogx(f, [Xsm Ysm*fact]);
grid on; set(gca,'xlim',[10 24e3])
ylabel('dB')
title('Audio Injector wm8731 card, very flat')
subplot(212)
semilogx(f, [Xsm Ysm*fact]);
grid on; set(gca,'xlim',[10e3 24e3])
ylabel('dB')
xlabel('f (Hz)')
title('3 dB point above 22 kHz')

print -dpng /tmp/audioinjector.wm8731.analysis.png
end

[thumbknuckle]

So there is a built in filter that is enabled by default?

[flatmax]

I think so - is it enabled by default ?

Matt

[omnyacoustics]

Are these results representative of the Zero model as well? If not, any place I can view those?